To start with I should say that I think any model ship is a good model ship, particularly if its builder has had fun building it. For non-commercial model builders the choice of subject for a model is a matter of personal taste and degree of difficulty.
For a commercial model-builder, it's a bit different, isn't it. The choice of subject matter has to be such that it attracts enough sales to recover the cost of tooling-up and then make a profit. I would have thought a model of Q3 was a bit esoteric to generate significant sales but maybe there's a big collector's market that I don't know about.
As for producing a model of a ship that never existed, I suppose if it's a (would-have-been) ship of sufficient historical significance then why not make a model of it? Perhaps what Lynda and June are alluding to is the fact that a model is a smaller or larger representation of a prototype, so how can you have a model without a prototype? Well, the drawings of Q3 were developed to the point that a three-dimensional representation of the design was realisable so maybe that passes for a prototype.
Call me a pedant, but I couldn't help noticing that the lines of the master's hull are pretty full at the waterline, much fuller than QE2 and the shape that Q3 would have been. But having said that I've also noticed that the model kits of QE2 show a much fuller hull than she actually has. QE2 was significant for, amongst so many other things, not having a parallel mid-body to her hull which gave her a very fine entry and a very fine run aft.